The narrow bezel display technology is gradually becoming the mainstream of flat panel display. TFT integrated driver circuit (Gate-driver on Array, GOA) design is essential for realizing narrow bezel display. Due to adopting of GOA circuit, the number of source and gate driving chips and the related connections be reduced, thus not only the TFT display panel becomes compact and attractive due to significant decrease of bezel size of the display, but also the following module and packaging process can also be simplified. Consequently, the manufacturing cost of the display can be greatly reduced, and the resolution and reliability of the TFT display panel can also be improved.
How to improve its reliability is the key problem in the design of TFT integrated gate driving circuit. For GOA circuits, the TFTs for low-level-holding experience long time stressing with positive gate voltage. Thus the threshold voltage of the low-level-holding TFTs is prone to increase with the operating time. When the threshold voltage shift of the low-level-holding TFTs reaches certain amount, GOA circuits fail. For application of desktop monitor or TV panel, the reliability of TFT integrated line scan driving circuit is prominent particularly due to increased operating time compared with that of other display applications. However, previous designs are mainly focusing on improvements in device characteristic. The reliability of the GOA circuit can be enhanced by suppressing shift of device's electrical characteristics.
In a variety of previous GOA circuits, constant gate biasing mode is popularly used for the low-level-holding TFTs. However, according to the GOA circuit operating principle, constant-biasing method leads to the low-level-holding TFTs biasing with high gate voltage with unnecessarily long operating time, thus the threshold voltage shift of TFT is large and the life of the circuit is difficult to be prolonged.
As showed in the FIG. 14, almost all the GOA circuits include 3 basic modules: input, output and low-level-holding module. The T100 is input device; the T200 is output device, which outputs the line scan pulse signal; the T300 and T400 are low-level-holding devices, whose gate input voltage is high as a constant value. There are usually overlap capacitance between the gate electrode and the source-drain electrode, as CGD of T200 as showed in FIG. 14. During the low level holding period, when clock signal of the T200 drain switches from low level to high level, the voltage potential of gate electrode of T200, which is originally at the low level, will rise due to the coupling of CGD. If the feed-through induced voltage-rising trend cannot be well suppressed, T200 will switch to the sub-threshold region or even the on region, which will lead to a very large charging current at the output electrode and the output can't be maintained low. At this time, T300 and T400 are turned on, and the gate of T200 and output electrode can be pulled-down, respectively.
But one of the main problems is that the threshold voltage of TFTs is prone to increases with time under electric stress, which leads to the degradation of the conducting ability. When the threshold voltage increases from the initial value (such as VTH0) to a certain critical value (such as VTHC), T300 and T400 is no longer being able to maintain the low level of gate electrode of T200 and the output, and the circuit starts to malfunction.
According to the operation principle of the mentioned GOA circuit, for normal operating, the gate-to-source over-drive voltage (the difference between the gate-source voltage and the threshold voltage) of T300 and T400 is required to be slightly larger than the difference (VTHC−VGL−VGH). Here the VGH and VGL represents high/low level of clock signal, respectively, for driving T300 and T400. But in previous GOA circuits, the level of the driving clock signal of the low level holding device is constant. Thus for most time, especially for early operating time, over-drive voltage of T300 and T400 is much greater than the difference (VGH−VTHC−VGL−). For example, VGH, VTHC, VTH0 and VGL are 25V, 20V, 3V, and 0V, respectively. In the early stage of the circuit operating, the required gate over-drive voltage of T300 and T400 is only 5 V, but the actual value of the over-drive voltage (VGH−VTH0−VGL) reaches 22 V. Both theoretical and experimental results show that the shift of the threshold voltage accelerates due to the increase of the driving voltage. To sum up, due to the constant biasing method, in the previous GOA circuits, the high level of the clock signal is constant, threshold voltage of relevant TFTs shift too fast and circuit life is difficult to be prolonged.